Switch



April 1941- c. G. KRONMILLER 21,765

SWITCH 2 Sheets-Sheet 1 Original Filed Aug. 6, 1956 L in 151M101 gar? GJfronmz'ZZer v April 8, 1941 C. G. KRONMILLER SWITCH 2 Sheets-Sheet 2 Original Filed Aug. 6, 1936 lllllll' l" Inventor 8611 2 6. Krorzmz'ZZer Attorney R'eiuued Apr. 8, 1941 swrrcn Carl G. Kronmlller, Minneapolis, Minn assignor to Minneapolis-Honeywell Regulator Company,

Minneapolis,

Minn, a corporation of Delaware Original No. 2,165,801, dated July 11, 1939, Serial No. 94,529, August 6, 1938. Application for reissue April 13, 1940, Serial No. 329,550

Claims.

My invention relates to a switch and'more particularly to a switch controlled by a condition responsive means.

It is common to employ for various control purposes a switch which will remain in closed position only when some controlling position has a desired value. A particularly common use for such a switch is in a safety pilot" in a fluid fuel burner control system when the switch will remain closed only when the pilot is burning. In

certain of such switches employing. an electromagnetic holding means, a manual operating means is provided which is eifective to initially close the switch, the switch remaining closed only if the holding means is energized. In prior art devices of this type, ithas been possible to close the switch momentarily by the operating means before the electromagnetic means is enenergized. In other words, each time the operating means is actuated the switch is temporarily closed during such attempt to permanently close the switch. This may have a very definite disadvantage. For instance, in the use just disclosed, whereinthe switch operates as a safety pilot, the closing of this switch may under certain circumstances admit fluid to a fuel burner which fuel, by-reason of the fact that the pilot is unignited, will itself not be ignited. The result is that each actuation of the operating means admits a certain amount of unignited fuel to the burner which, when the pilot is finally ignited, may lead to disastrous results.

' Moreover, in certain cases where an attempt has been made inswitches employing other types of holding means to prevent the operating means from changing the circuit controlling position of the switch until the holding means is effective, no means has been provided for preventing the changed again upon an accidental actuation of the operating means after the holding means is effective. Thus, referring again to the use of the switch as a safety pilot, if it is possible to open the switch by actuating the operating means after the burner has been started, the result will be that theburner is extinguished momentarily and if at the same time the pilot burner is extinguished the release of the operating means will cause gas to be admitted to the main burner until the device responsive to the pilot flame has cooled off sufficiently to permanently open the safety pilot switch.

an object of the present invention is to provide a switch controlled by an electromagnet and an operating means for the switch which is effective to move the switch. to a desired controlling position only when the electromagnet is energized.

A further object of the present invention is to provide a switch of the type set forth in the previous object in which the electromagnet is energized by a thermocouple.

A further object of the present invention is to provide a switch having an actuating means which is effective to move the switch to a desired controlling position only when a desired condition exists and is ineffective to move said switch from said position.

Other objects of the present invention will be apparent from a consideration of the specification, claims and drawings, of which Figure 1 is a sectional view of one form of my improved switch.

Figure 2 is a view of the contact blade structure and the insulating base supporting said blade structure, said view being taken along the line 2-2 of Figure l.

Figure 3 is a view of the portion of the switch apparatus during an intermediate stage of the operation thereof.

Figure 4 is a view of the switch mechanism in its switch-closed position.

Figure 5 is a detail view taken along the line 5-'-5 of Figure 4.

Figure 6 is a schematic view of one application of the improved switch.

Figure 7 is a section view of a modified form of the switch, the section being taken along th line 1-1 of Figure 9.

Figure 8 is a sectional view of the switch shown in Figure '7, the section beingtaken along the line 8-8 of Figure 7.

Figure 9 is a sectional view of the modified switch along the line 9-8 of Figure 7.

Figure 10 is a detail view of two of the contact carrying members, and

Figure 11 is a detailed view of. a portion of the switch operating mechanism with. the switch members in a position assumed when the contacts are in a second circuit controlling position, the section being taken along the line llll of Figure 9.

Referring to Figure 1 of the drawings, the switch is mounted in a housing In to which is secured an insulating plate ll. Mounted on insulating plate II are contact carrying arms l2 and II, which are secured thereto by terminal posts it and i5 respectively.

As best shown in Figures 3 and 4, the contact carrying arm I! extendsdiagonally upwardly plate I I.

ly upwardly from the base and then horizontally,

terminating in a downwardly extending leg 2| which is bent at 22 to provide a portion lying under the contact ll of member l2. Secured to this portion is contact 23 which is adapted to V cooperate with contact ll. Secured to the horizontal portion of arm II is a relatively rigid strip 24 which extends beyond the strip II to constitute a horizontal continuation of the same. The

. arm I3 is of resilient material which resists move ment of the arm from the position shown in Figure1.

Also secured to insulating base lllis a resilient strip which has secured thereto a relatively rigid strip which overlies the strip 14. and is adapted to cooperate with the same in a manner to be subsequently disclosed. The strip 25 tends to move upwardly by reason of its resilience.

A lever is pivotaliy secured to the insulating A portion of this lever is channelshaped and is provided with side walls II and a base portion 32, which base portion 32, extends beyond'the side walls II. The arm}. is secured to plate I I through a pivot pin it which, as shown in Figure 5, is non-rotatably secured to plate l I by a set screw. A spring 38 has one end extending through the pivot pin 33 and has its other end engaging the base portion 32 of arm ll so as to urge lever 30 in a counter-clockwise direction. A roller 31 is secured between the two side portions ii of lever ill and cooperates with the base portion l8 oi the U-shaped portion of contact carrying arm l2. Also mounted on pivot pin 33 is a U- shaped arm 39 which is provided with an end portion 40 which is inclined in such a direction that when it lies under the strip 28 as shown in Figure 4, it is substantially parallel therewith. Arm

38 is provided with. two ears ll, as best shown in Figure 5. Two springs 43 are mounted on pivot pins 33 and cooperate with ears Al and with the side walls ll of lever 30 so as to urge ears no: member 39 into engagement with the upper edge of arm 30. The result is that a strain release connection between arm 39 and lever 30 is provided;

As shown in Figure 1, a u-shaped bracket 48 is secured to the insulating plate I I through ears 4 which project through the plate II and may be secured thereto by crimping or any other suitable manner. The bracket 45 is provided with a cen-' tral portion 41 which is enlarged so as to have a cylindrical housing member 48 secured thereto.

Located in this housing 48 is a push button ll which is provided with a collar intermediate its lever 3| shown in Figures 1 and 4.

from the base and then horizontally. Secured through an aperture in the base portion 32 of arm III. A flange I. is provided at .the outer portion of stem l1 and a spring II is located between armature I! andmember IO and normally serves to urge the armature I to the position. relative to The spring II and flange I. serve to provide a strain release connection between armature I5 and arm 82.

, The armature I6 is designed to cooperate with an electromagnet II which comprises a core II and a winding 62. The core II is preferably of material of high magnetic permeability. The windings .2 are energized by a thermocouple It. The thermocouple l is provided with the usual members III and 'H of different thermoelectric characteristics which are jointed at I! to provide the hot Junction of the thermocouple. While members II and II may be of any suitable material having desired thermoelectric characteristics. it has been found particularly'desirable to make the outer member ll of a suitable stainless steel two ends. The push button 49 terminates in a reduced stem portion Bl which" extends through an aperture in the portion 41 of bracket ll. A spring I? is located between member 41 and collar 50 and serves to urge push button 49 outwardly. The push button is so located that the stem I is adapted to engage strip 26 upon the push button being moved inwardly. A cap I! is threaded on housing 48 and is adapted to prevent the accidental displacement of push button 0. A cover N enclom the upper portion of the switch mechanism.

Associated with lever 30 is an armature ll which is provided with a stem 51 extending which will resist the heat of a pilot flame and the inner member of Constantan, an alloy consisting of approximately 50% nickel and 50% copper. The innermember II is secured to a conductor II which is insulated as at II. The outer member II is connected-to a copper reducing sleeve II which in turn is connected to a copper tubing IS. The copper tubing 16 is electrically connected through a sleeve II with one end of the winding 02 of the electromagnet. The sleeve 11 also serves to secure tubing 1' to the casing ill. The inner conductor II is secured to the other end of the winding 02 through a coupling sleeve II which is insulated from sleeve 11 through a suitable insulating bushing 8!.

In order for the extremely small electro-motive force which is generated by the diil'erence in temperature between the hot junction 12 and the cold junction, to produce a suflicient holding action in the electromagnet, it is necessary that the resistance of the circuit energized by the thermocouple be extremely low. In order for this to be true it is necessary that conductors ll, copper tubing It and the conductor comtituting the winding 02, be of a relatively large size so as to alter little resistance to the flow of current.

As previously stated, one application for which the switch is particularly designed. is that of a safety'pilot in a fuel burner control system. Such an application is shown schematically in Figure 6 the valve 81 may be of a motor operated type. A

pilot burner 8l is associated with the main gas burner and it is desired that it be impossible to open the main gas valve 81 unless this pilot is burning. The improved switch of the present invention is designated in its entirety in this figure by the reference numeral II. The motor of valve 81 is under the joint control of this switch and a thermostatic switch" of any suitable construction.

A stepdown transformer "supplies power for the operation of the valve. This transformer comprises a high voltage primary 83 connected to line wires 00 and a low voltage secondary 94. The secondary O4 is connected through a conductor II with a fixed contact of the thermostat 8!. Conductor OI connects the other terminal of the thermostat with one terminal of switch DI and a conductor Ill leads from the other ter- 'ment with arm 30.

minal of the switch 90 to one terminal oi a valve operating mechanism 01' valve 81, the other terminal of which is connected to secondary 94 by a conductor Il. It will be readily] apparent that when switch 90 is open it is impossible to establish a circuit to the alve operating mechanism of valve 01 even though thermostat ll be closed; It is accordingly imperative in order for the valve 81 to be opened that switch 90 be closed. For a reason which will be more apparent from the subsequent description of the operation of the switch, it is impossible to close switch 90 unless the pilot 99 is burning.

' The switch is shown in Figure 1 in the position assumed when .the pilot is extinguished. Under these conditions, the biasing action of spring 36 urges the lever 30 in a counter-clockwise direction and due to the action of roller 31 pressing against portion I9 of contact carrying arm I2,

the latter arm is held in the position shown in Figure 1 against the action of its own resiliency, which tends to urge it downwardly. The contact arm I3 tends to assume the position shown in Figure l by reason of its inherent resiliency. The resiliency of member 25 tends to hold the rigid strip 26 substantially in engagement with push button 49. Due to the fact that the energization of the electromagnet by thermocouple 64 is relatively small, armature 55 will not be moved by the magnet downwardly into engagement with the magnet from the position shown in Figure 1. It is necessary if there is to be any co-action between the electromagnet 50, and armature 55 for these two members to be brought into engagement. The push button mechanism is for this purpose.

Upon push button 49 being moved inwardly with respect to plate II, the plate 26 will first engage plate 24 and will then urge arm I2 downwardly through its engagement with the outer leg I9 of arm I2. Due to the fact that the contact 23 is rigidly secured with respect to contact arm I3 and that the spacing of contact carrying arms I2 and I3 is fixed when contact arm I3 is being moved downwardly by reason of the leg l9, the downward movement of these members by the actuation of push button 49 does not cause engagement of contacts I1 and 23. Upon push button 49 being moved completely inwardly the parts assume the position shown in Figure 3. It will be noted that the arm 39 has moved against the end of plate 26 where. it is resiliently held by the action of springs 43 which, as previously stated. tend to urge ears 4| into engage- It will be further noted that the downward movement of contact arm 30 has caused armature 55 to be moved into engagement with the core6l of electromagnet 50. As previously stated, the spring 59 functions to provide a strain release connection between arm 30 and armature 55 so as to insure a firm seating of armature 55 with core 6| of electromagnet 56.

If the pilot burner is notignited so that electromagnet 56 is not energized, the release of push button 49 will cause the parts to all return to the position shown in Figure 1. It will be noted that during this entire action, contacts I1 and 23 are maintained spaced so that it is impossible to even momentarily close the circuit to the gas valve 81.

I f, however, when the parts are moved to the .p' tion shown in Figure 3 by reason of push bu ton 49 being moved inwardly, the pilot burner is lighted so that electromagnet 50 is energized,

armature 55 will be held in engagement with the electromagnet. Under these circumstances the arm 30 will not be permitted to return to the position shown in Figure 1 but will move in a counter-clockwise direction merely until the portion 32 thereof engages the fiange 58 as shown in Figure 4. Due to the fact that the switch arm I2 is biased downwardly by reason of its own resilience, its upper motion will terminate as soon as the portion 32 of lever 30 engages flange 59. Arm l3 will, on the other hand, continue to travel upwardly by reason of being biased upwardly because of its resilience with the result that upon push button 49 reaching its'uppermost position the various parts will be in the position shown in Figure 4, wherein contacts I! and 23 are in engagement. It will be noted that the actuation of push button 49 results in contacts I1 and 23 being brought into engagement only when the pilot is ignited.

Referring to Figure 4, it will be noted that the portion 40 of arm 39 lies underneaththe strip 26. Referring to Figure 3 it will be noted that the only thing which tends to prevent arm 39 from rotating in a clockwise direction with respect to lever 30 is the engagement of this arm with the end of strip 25. When this strip moves upwardly due to the push button being released, springs 43 will be effective to move arm 39 to the position shown in Figure 4. If it is now attempted to change the position of the switch by actuation of push button 49, rigid strip 26 will never be moved into engagement with strip 24 due to the fact that it will first engage arm 40 and further downward movement of strip 26 will be prevented. The result is that it is impossible to even momentarily open the circuit to the actuating means of gas valve 81. For the reasons previously pointed out, this is a considerable advantage over prior structures.

In Figures 7 to 11 of the drawings a modified form of the switch is shown. The principal difference between the modified switch and the one previously described is that this switch is partly adapted for. the control of a three-wire circuit as distinguished from a two-wire circuit for which the previously described switch was adapted. There are also certain diilerences in constructional detail which are of general applicability to either a two-wire or a three-wire type of switch.

Referring to Figures '7 and 8, a base plate is designated by the reference numeral I03. Located above this plate I03 is an insulating plate I04 which is supported on base plate I03 by spacers I05. A bracket I06 of inverted U-shape is located upon the insulating plate I04. Screws I01 serve to connect plate I03, insulating plate I04 and bracket I06 to the spacers I05,and to secure these members together in assembled relation.

Also mounted upon insulating plate I04 are contact carrying arms IIO, III and H2. -These contact carrying arms are, as best shown in Figures 9 and 11, secured to the plate I04 through terminals H4, H5 and IIS respectively.

The contact carrying arm I I0 is relatively rigid. This arm after leaving the terminal I I4 first extends vertically and then terminates in a horizontal portion II8. This horizontal portion is enlarged adjacent its midpoint and extending through this enlarged portion is a circular opening 9. Secured at the outer end of horizontal portion H0 on the underside thereof is a contact I20.

The contact arm II I is of more or less resilient material. and is so disposed that it tends to move downwardly by reasonoi its resiliency. This arm, as bestshown in Figures and 11, is provided with an ear portion I2I which lies under the circular opening H9 01 contact carrying member IIO. This ear portion I2I is provided with an aperture I therethrough which aperture is aligned with aperture II9. Arm III is further provided with a; vertically extending portion. I22 and extending from the last named portionis an ear I29 parallel to ear I2I. Secured to the opposite sides of ear I29 are contacts I24 and I25 which are aligned in a vertical direction withcontact .I20. Contacts I20 and I24 are adapted'to cooperate with each other and are shown in engagement in Figures 7 and 8,

The contact carrying arm H2 is also of resilient material and is so iormed that the outer portion 01 this arm tends to move upwardly by reason of the resiliency of the arm.. This arm is provided with a portion i29 which extends diagonally upwardly and terminates in a horizontal.

portion to the upper side 01' which a contact I21 is secured. The horizontal portion is formed so as to provide an ear'I29 which is adapted to be aligned with apertures I I9 and I29.

Secured to the upper horizontal portion oi bracket I09 is a cylindrical housing member I30 This member houses a push button I9I corresponding in function to push button 49 oi .the previously described species. Pushbutton I9I is provided with a shoulder I92 and is threadedly connected to a cylindrical memer I92 oi insulating material which is located on the opposite side of bracket I09 from the portion of push button III which is manually operated. A spring I94 is located between member I09 ,and shoulder I92 and serves to bias the push button outwardly. The outward movement oi push button I3I is limited by reason of the engagement of the cylindrical insulating member I39 with bracket I09. The cylindrical member I99- extends through opening I I9 and is adapted to engage the ear I28 of contact carrying arm I I2.

Secured to the base plate I03 is an electromagnet housing I91. Located within this housing I41 is an electromagnet I99. A member I99 functions as a closure for the electromagnet housing, as a guide for the armature cooperating with the electromagnet, and as a spring bearing member. This member is provided with a collar I which is clamped between a shoulder of housing I91 and the base plate I09. Extending downwardly from the central portion of member I99 are two spaced members I which function as guides for an armature I42 which is adapted to .cooperate with the'elect-romagnet I99. The armature I 42 is securedto a stem I49 through a clip I44. Secured to the upper end of stern I49 is a cup-like member I45 as best shown in Figure 11. An insulatingstud I41 is threaded into the cup-like member I45 and a split ring I49 is located between these two members and serves to secure the same to stem I49. Insulating stud I41 is provided with a reduced portion I49 at its upper end which extends through the opening I29 in contact carrying member III. A spring I49 bears at one end against member I45 and at the other end against the base of a recessed portion I50 oi member I99, which recessed portion serves as a bearing for spring I49. The spring I49 serves to bissmember' I45and consequentiystud I41 upwardiysoastomaintaincontactcarrying member III in an upper position such'ssshown cues in Figure 'I wherein contacts I20 and I24 are in engagement.

A cover I55 extends over the switch mechanism and is secured in placeby screws I56 serving to connect the cover with bracket I09. Due to the fact. that the thermocouple structure is thesame as that shown in the previously described species, aseparate showing 0! the same in connection with the present species has not been made. It is furthermore believed unnecessary to repeat the description of the various details of the electromagnetand the method of connecting the leads tromthe thermocouple to the electromagnet. l

The various elements oi" the switch are shown in Figures 7 and 8 in the position which they assume when the electromagnet is deenergiud due to the absence of a flame adjacent the thermocouple. In this position contacts I20 and I24 are in engagement. when it is desired to bring contacts I25 and I21 into engagement. the push button I9I is pressed downwardly. The actuation 0! push button I3I causes insulating member I 22 to move downwardly. The downward movement of insulating member I33 causes its engagement with ear I29 of contact carrying arm I I2, causing the latter member to move downwardly. At the same time the downward motion of member III is transmitted through ear I29 tostud I41, causing the latter member to also move downwardly. As previously stated, contact carrying arm III is biased downwardly and is held in the position shown in Figure 11 only by reason of the biasing action of spring I49 acting through members I45 and I41 to move arm III upwardly. Upon the downward movement of member I41, contact carrying arm III is iree to move downwardly, moving down substantially simultaneously with contact carrying arm I I2. As a result both members are shifted in position but contacts I25 and I21 are not allowed to mo e into engagement.

The downward movement of stem I49 causes armature I42 to be moved into engagement with electromagnet I39. Due to the fact that armature I42 is now in direct engagement with magnet I29, the latter will be eflective to hold the armature ii the thermocouple is being heated. Ii the thermocouple is not being heated the release or push button I3I will restore the parts to the position shown in Figures 7 and 8, thus causing re-engagement oi the contacts I20 and I24 without having at any time eflected engagement of contacts I25 and I21. Due to the fact that in a normal use of this apparatus in three-wire cir cults, the engagement of contacts I25 and I21 would efiect the operation of the iuelsupplying apparatus, it is highly important that these contacts not be engaged even momentarily for a reason similar to that set forth in connection with the previously described species oi the apps.- ratus. Y

1!, however, the pilot is burning properly and armature I44 is held inengagement with magnet I28, stud I41 is held in its lowermost position as indicated at Figure 11. Due to the fact that as previously mentioned contact carrying arm III is biased downwardly, this arm will remain in its lowermost position as indicated in Figure 11.

' scribed species, the engagement of the contacts which cause operation or fuel ifeeding menu or which bring about a normal running condition of the system being con-trolled, is not effected by operation of the initial resetting means unless the electromagnet is energized by reason of the existence of, a desired condition such as the ignition oi. the pilot burner. Moreover, this ob- ,lect has been accomplished in both species with an extremely simple mechanism which has a minimum number of moving parts.

While certain specific embodiments oi the present invention have been shown and described, it is to be understood that these are for purposes of illustration only and that my invention is to be limited only by the accompanying claims.

I claim asmy invention:

1. In combination, a switch movable between first and second controlling positions and biased to a first position, holding means comprising an electromagnet for holding an element associated with said switch in a position wherein said switch is biased to said second controlling position, actuating means for moving said switch, without moving the same to said second controlling position, to a position wherein said element is held by said holding means it said electromagnet is energized, said biasing means being operative upon further movement oi. said actuating means to move said switch to said second controlling position, and means to prevent movement of said switch from said second circuit controlling position by said actuating means.

'2. In combination, a switch movable between first and second controlling positions, means biasing said switch to said first position, holding means comprising an electromagnet tor holding an element 01' said biasing means in a position wherein the latter is effective to bias said switch to said second controlling position, actuating means for moving said element of said biasing means, without affecting the circuit controlling position of said switch, to a position wherein said holding means is effective ii energized, and means effective upon said switch being moved to said second controlling position to prevent movement of the same from said position by said actuating means.

3. In combination, a pair of resilient contact carrying members tending by reason of their own resilience to move to a first circuit controlling position, biasing means normally acting on said contact carrying members to urge the same against their resilience into a second circuit controlling position, holding means for retaining said biasing means in a position wherein said contact carrying members are not affected thereby, actuating means operative upon movement thereof in one direction to move said biasing 'means while preventing said contact carrying members from moving to said first circuit controlling position, into a position wherein said biasing means maybe retained by said holding means if eiiective, said members being operative upon further movement or said actuating means to move to said first circuit controlling position by reason of their own resilience.

4. In combination, a pair of resilient contact carrying members tending by reason of their own resilience to move to a first circuit controlling position, biasing means normally acting on said contact carrying members to urge the same against their resilience into a second circuit controlling position, holding means for retaining said biasing means in a position wherein said contact carrying members are not afiected thereby, actuating means including a reciprocable member operative-upon movement "01 said-member in one direction to move said biasing means while preventing said contact carrying members irom moving to said first circuit controlling posierative upon movement of said reciprocable member in the opposite direction to move to said first circuit controlling position by reason or their own resilience.

. 5. In combination, three contact carrying members, a first of which is substantially immovable and a second and third of which are movable, said second member being biased into a contact making position with respect to said first member, reciprocable means for moving said second 7 and third contact carrying members without changing the relation of the contacts carried thereby, and means operable when said members are so moved by actuation oi. said reciprocable means in one direction to hold an element associated with one 01' said members so that upon actuation of said reciprocable means in the opposite direction, only one 01 said two contact carrying 1 members returns in the direction of its original position whereby the contacts carriedby said second and third members are brought into engagement.

6. In combination, a switch normally resting in a predetermined circuit controlling position, means to move said switch back and forth without changing its circuit controlling position, an electromagnet member and an armature member, one of said members being operated by said means to bring the same adjacent the other oi said members, means to move said switch to another circuit controlling position when said members are held adjacent each other by reason of the energization of said electromagnet upon movement of said switch moving means to a position in which said members would otherwise be moved apart, and means to prevent further actuation 01' said switch moving means from changing the circuit controlling position of said switch.

'1. In combination, a pair of resilient contact carrying members tending by reason of their own resilience to move to a first circuit controlling position, an element adapted to engage one of said contact carrying members and biased so as to hold said member out of said first controlling position with respect to said other contact carrying member, an armature secured to said element, an electromagnet adapted upon enerfirst and second controlling positions and biased to a first position, selectively operable holding means for holding an element associated with said switch in a position wherein said switch is biased to said second controlling position, actuating means for moving said switch, without moving the same to said second controlling position, to a position wherein said element is held by said holding means i! operative. said biasing means being operative upon further movement of said actuating means to move said switch to said second controlling position, and means to ing said biasing means from affecting the posi-' tion or said first member, said holding means comprising an electromagnet and an armature,

v and a reset member operable when moved in one direction to force said other member into'engagement with said first member and move said first member to the position it normally occupies when the contacts carried by said members are engaged, said first switch member remaining 'in said position when said reset member is moved in the other direction if said armature is retained by said electromagnet and said other switch member moving back by reason of its resilience away from n nes said first member to contacts.

10. In combination, a pair of resilient contact carrying members, one oi said members having a contact carrying portion extending around the contact carrying portion of the other member so that the contacts carried by said members are moved into engagement when said members are moved apart, biasing means urging afirst of said members towards the other so as to urge said contacts apart, holding means i'orpreventin'g said biasingmeans from aflecting the position of said first member, said holding means comprising an electromagnet and an armature. and a reset member operable when moved in one direction to reset said armature with respect to said electromagnet and to force said other member into engagement with said first member and move said first member to the position it normally occupies when the contacts carried by said members are engaged, said first switch member remaining in said position when said reset member is moved in the other direction if said arma-v ture is retained by said electromagnet and said other switch member moving back by reason of its cause engagement of said resilience away from said first member to cause engagement of said contacts.

cam. G. 

